In order to manufacture a cast strand free from a surface defect by means of, for example, a vertical type continuous casting machine, it is essential to minimize variations in the surface height of molten steel in a mold. For this purpose, it is necessary to continuously and accurately measure the surface height of the molten steel in the mold. The surface height of the molten steel in the mold can be determined by measuring the distance between the tip of a probe arranged substantially vertically to the surface of the molten steel at a distance therefrom and the surface of the molten steel.
An example of the method for measuring the above-mentioned distance is disclosed in Japanese Patent Provisional Publication No. 57-192,805 (hereinafter referred to as the "prior art"). The prior art is described below with reference to FIG. 1.
As shown in FIG. 1, an AC power source 1 impresses an AC voltage having a prescribed frequency and a prescribed amplitude onto a positive feedback amplifier 2. The positive feedback amplifier 2 has a feedback path comprising a probe 3 and an AC voltage amplifier 9. The probe 3 comprises a bobbin 4, a primary coil 5 provided coaxially with the bobbin 4 at the center thereof, and a pair of secondary coils 6 and 6' each provided at each of the end portions of the bobbin 4 coaxially therewith on the both sides of the primary coil 5 at equal distance therefrom. The probe 3 is arranged substantially vertically to the surface of molten steel 8 in a mold 7 at a distance therefrom. The primary coil 5 is excited by means of an output voltage (e.sub.out) of the positive feedback amplifier 2, whereby an AC voltage (e.sub.1) and an AC voltage (e.sub.2) are induced respectively in the pair of secondary coils 6 and 6'. The pair of secondary coils 6 and 6' are differentially connected to each other with equal winding turns. The AC voltage amplifier 9 amplifies a value of difference (e.sub.3) between the AC voltage (e.sub.1) and the AC voltage (e.sub.2) induced respectively in the pair of secondary coils 6 and 6', and feeds the thus amplified value of difference (e.sub.3') of the value of difference (e.sub.3) back to the positive feedback amplifier 2.
In the above-mentioned apparatus for continuously measuring the distance utilizing an eddy current of the prior art, an AC voltage having a prescribed frequency and a prescribed amplitude is impressed from the AC power source 1 onto the positive feedback amplifier 2. An output voltage (e.sub.out) of the positive feedback amplifier 2 is impressed onto the primary coil 5 of the probe 3. As a result, an AC magnetic field is generated by means of the primary coil 5. Lines of magnetic force of the AC magnetic field interlink with the pair of secondary coils 6 and 6' to induce an AC voltage (e.sub.1) and an AC voltage (e.sub.2) respectively in the pair of secondary coils 6 and 6'. Simultaneously with the interlinkage with the pair of secondary coils 6 and 6', the lines of magnetic force pass through the molten steel 8 to generate an eddy current in the molten steel 8. This causes generation of another AC magnetic field in the direction opposite to that of the above-mentioned AC magnetic field of the primary coil 5. As a result, part of the lines of magnetic force of the primary coil 5 interlinking with the pair of secondary coils 6 and 6' is offset, and the number of the lines of magnetic force thus decreases. The rate of this decrease in the number of the lines of magnetic force is larger for the lower secondary coil 6' than for the upper secondary coil 6, because the number of the lines of magnetic force generated by means of the another AC magnetic field of the eddy current and linking with the pair of secondary coils 6, 6' is larger for the lower secondary coil 6' than for the upper secondary coil 6. This results in a difference (e.sub.3 ) between the AC voltage (e.sub.1) and the AC voltage (e.sub.2) induced respectively in the pair of secondary coils 6 and 6'. Since the pair of secondary coils 6 and 6' are differentially connected to each other, the value of difference (e.sub.3) between the pair of secondary coils 6 and 6' is continuously impressed onto the AC voltage amplifier 9. This value of difference (e.sub.3) is amplified by the AC voltage amplifier 9. The amplified value of difference (e.sub.3') is continuously fed back to the positive feedback amplifier 2.
The amplified value of difference (e.sub.3') varies in response to the distance (l) between the tip of the probe 3 and the surface of the molten steel 8, and the output voltage (e.sub.out) of the positive feedback amplifier 2 varies in response to the amplified value of difference (e.sub.3'). It is therefore possible to continuously measure the distance (l) between the tip of the probe 3 and the surface of the molten steel 8 by continuously detecting the output voltage (e.sub.out) of the positive feedback amplifier 2.
The above-mentioned prior art has the following drawbacks. When there is no temperature difference between the pair of secondary coils 6 and 6', the increments by thermal expansion of the respective cross-sectional areas of the pair of secondary coils 6 and 6' are equal to each other. Therefore, the amplified value of difference (e.sub.3') between the pair of secondary coils 6 and 6' fed back to the positive feedback amplifier 2 shows no variation. When measuring the surface height of the molten steel 8 as mentioned above, however, the lower secondary coil 6' closer to the surface of the molten steel 8 is heated to a higher temperature than the upper secondary coil 6. As a result, the cross-sectional area of the lower secondary coil 6' becomes larger than that of the upper secondary coil 6. The amplified value of difference (e.sub.3') fed back to the positive feedback amplifier 2 therefore varies in response to the difference in cross-sectional area between the pair of secondary coils 6 and 6', resulting in a measuring error of the above-mentioned distance (l).
Under such circumstances, there is a demand for developing a method and an apparatus which enable to continuously and accurately measure the distance between a high-temperature conductive object of measurement and the tip of a probe which comprises a primary coil and a pair of secondary coils provided on the both sides of the primary coil, even when a difference in temperature between the pair of secondary coils of the probe is caused by the heat of the high-temperature object of measurement, without being affected by the above-mentioned difference in temperature, but such a method and an apparatus are not as yet proposed.